It is extremely important to obtain uniform digestion in pulp manufacturing processes, both from the viewpoint of economy and with respect to the quality of the pulp obtained. The reasons why non-uniform digestion is obtained vary in accordance with the type of process being utilized. In Swedish patent specificaton No. 345,885, some of the most significant factors which influence the homogeneity during periodic digestion are listed, and a method of eliminating the negative effects of one of the most significant of such factors, namely, the circulation of the cooking liquid.
The technique employed in accordance with that application is the heat boiling-up, and in accordance with that experiences in mill installations, extremely positive effects upon the homogeneity during digestion have been demonstrated. In accordance with this method, boiling of the cooking liquid is effected by rapidly decreasing the pressure in the digester in order to effect upward flow therein, and temperature balances in the digester. This is effected either by violent gassing or by the intake of cooling liquor to the top of the digester.